1. Technical Field
The present invention relates to a fixing device and an image forming apparatus using the same.
2. Related Arts
As the device for fixing the unfixed toner image onto the recording medium, the heating/fixing roller heated by a halogen heater arranged in the roller, a pressure roller pressed against the heating/fixing roller by a coil spring, or the like to form a nip portion between the rollers, and a recording medium, onto which the toner is put, is heated, pressed by passing the recording medium through the nip portion.
In case various improvements such as speed up of the printing, size reduction of the housing, improvement in reliability of the product, energy saving, cost reduction, and the like are introduced, the system for bringing the above rollers into contact with each other with pressure has the problems described as follows.
In case the speed up of the printing that is important out of them is intended, a carrying speed of the recording medium is inevitably increased. At that time, in order to ensure the fixing performance of the toner equivalent to that obtained before the carrying speed is increased, the equivalent thermal energy must be applied in the fixing step. However, the measure to apply such energy by increasing a heat value of the heater goes against the energy saving. Therefore, it is supposed that the heat value of the heater is not changed.
First, it can be considered that a contact area is increased by enlarging a diameter of the roller. That is, a contact distance in the transferring direction along which the toner on the recording medium is heated is made longer. Accordingly, a heating time can be ensured. However, an increase in diameter of the roller goes against a downsizing of the housing.
Next, the method of increasing a load value by lowering an elastic modulus of the roller surface can be considered. In order to lower the elastic modulus of the roller surface, either a thickness of a surface layer of the roller is increased or a rigidity (Young's modulus) of the member must be lowered. However, when a thickness of the surface layer of the roller is increased, a heat capacity of the overall heating roller is enhanced, so that a heating time required to raise a surface temperature of the heating roller up to a predetermined temperature is prolonged. As a result, a printing time is also prolonged. Also, there is the method of maintaining a temperature of the heating roller by supplying a heat as needed during the print standby, but such method is contrary to the energy saving.
In contrast, when the load value applied to bring the rollers into contact with each other with pressure is increased, an amount of deflection of the roller is also increased and thus a pressure difference is caused between the center portion and the end portion in the axial direction of the roller. In some cases the fixing performance of the toner becomes uneven. In addition, it is possible that an uneven pressure distribution causes a paper wrinkle, and consequently reliability of the device is degraded.
When an amount of deflection is reduced, a rigidity of the roller is increased, or the roller is formed into a crown shape. In the former case that a rigidity of the roller is increased, a heat capacity of the roller is increased and also a heating time is prolonged, which is contrary to the energy saving. In the latter case, the crown-shaped roller needs a large number of steps required to form the roller, and thus a reduction in cost is difficult. A pressure distribution precision in the axial direction becomes higher in sensitivity than a machining precision of the roller. Accordingly, the reliability may be degraded.
It is disclosed, by Japanese Patent No. 3298354, that the nip portion is formed by contacting the pressure applying member whose exit portion is elastically deformable to a rotatable heating roller, whose surface is elastically deformable, with pressure in a belt nip system for fixing the toner image, and that a toner image on a recording medium is pressurized, heated, and transferred between the nip portion to be fixed.
It is disclosed, by Japanese Patent No. 3282494, that the elastic layer is provided near the surface of the heating roller, and that the pressure applying member contacts the heating roller via the endless belt to curve along an outer peripheral surface of the heating roller and presses the heating roller with an almost uniform pressure distribution.
According to Japanese Patent No. 3282494, since the pressure applying member is provided to the endless belt such that a contact surface is formed from the inner side, a nip width can be formed longer than that obtained by the method that does not employ the endless belt, and such configuration can apply enough heat to the toner. Accordingly, such configuration can respond easily to a higher speed of the printing speed. A nip width can be formed longer than that obtained by the method, which does not employ the endless belt, not to increase a size of the overall device, and such configuration can respond to the downsizing of the overall device. In addition, a long nip width can be formed at a lower pressure than that in the method that does not employ the endless belt, and a thinning of the heating roller can be made easy. Also, a heating time required until a proper temperature for the fixing can be shortened and also the printing can be started quickly, whereby the energy saving effect can be improved. Further, since a distortion is provided to the elastic layer of the exit portion of the nip by pressing the elastic layer of the heating roller by the hard member, the self-stripping (self-releasing) performance can be increased higher than that in the method that does not employ the endless belt, and thus the reliability can be enhanced.
As the fixing device of the general image forming apparatus, as shown in JP-A=8-262903, for example, the belt nip system fixing device has already been proposed to fix the unfixed toner image transferred onto the recording medium. In this belt nip system fixing device, the nip portion is formed by pressing the pressure applying belt against the heating/fixing roller, a surface of which is elastically deformed, with pressure, and then the unfixed toner image is heated/fixed by causing the recording medium to pass through the nip portion.